5 Answers

I don’t know, but I do know that in Vietnam soldiers learned not to wear undershorts to allow the maximum circulation of air. It is extraordinary how many varieties of rot and fungus are endemic in a warm humid jungle environment, and a confined dark humid space, well enough said.

As per Wikipedia -
The origins of the phrase “go commando” are uncertain, with some speculating that it may refer to being “out in the open” or “ready for action”. Slate magazine’s Daniel Engber dates the modern usage to USA college campuses circa 1974, where it was perhaps associated with an old practice in the army in Vietnam War, who were reputed to go without underwear to “increase ventilation and reduce moisture.

Can’t help much, but I’ll try. When a person doesn’t wear underwear they “go commando.” The word “commando” only means a soldier trained in specific combat.
I’ve heard it said that Navy Seals, and probably other commando forces, don’t wear underwear because the sand on the beach gets into your underwear and causes chafing so they opt to “go commando.”